This invention relates generally to the formation of insulating layers during the fabrication of semiconductor integrated circuit devices, and more particularly to a method for reflowing such layers to form a smooth surface topography.
Bipolar and metal-oxide-semiconductor (MOS) devices frequently include phosphosilicate glass (PSG), silicon dioxide, or silicon nitride layers for insulation and passivation. These layers are typically thermally grown or deposited on silicon, polysilicon, or metal. During subsequent processing steps these insulating layers are patterned and etched using either plasma, reactive ion, or wet etching techniques. Such etchants, particularly plasma and reactive etches, usually produce vertical sidewalls, sharp edges and sharp corners in the etched layers, which tend to create step coverage problems during the formation of overlying layers. One known method for removing the sharp features in PSG is to subject the device to a high temperature in order to reflow the PSG. However, this process is satisfactory only for fairly high concentrations of phosphorus, i.e., 10-12 percent, in PSG. For lower concentrations, temperatures in excess of about 1,000 degrees C. are required to achieve reflow. In the case of very low concentrations of phosphorus, and for undoped oxide and nitride, this technique is unsatisfactory.
High temperature reflow can also not be utilized when the insulating layer overlies a layer of low melting point metal, for example, aluminum or an aluminum-copper alloy. Even high melting point metal layers usually suffer degradation during high temperature processing. Such heat treatment steps normally last 30-60 minutes, which is long enough to cause substantial diffusion of implanted or diffused impurities in silicon and polysilicon. This diffusion is particularly undesirable in very large scale integrated circuit devices.
If the device is heated for a shorter period of time, higher temperatures are needed to cause reflow and the time-temperature product still remains unacceptably high. Further, even where a high temperature step is acceptable for reflowing a PSG layer, the concentration of phosphorus required in the PSG is so high that the phosphorus will eventually react with moisture in the air to form phosphoric acid which corrodes the metallization in the circuit and frequently causes device failure.
In addition to the need for reflowing edges and corners, heating or annealing of oxide and nitride layers is advantageous for reparing damage caused during implantation and for improving the structural integrity of oxides grown on polysilicon, i.e., interlevel oxides (ILO). One technique for improving the ILO is to laser anneal the polysilicon prior to oxide growth. The undesirable heating of the underlying layer is still a problem, however.
There is thus a need for a method for heating the insulating layer to be reflowed while maintaining the other regions of the device at a relatively low temperature.